CN115552049B

CN115552049B - Duplex stainless steel and duplex stainless steel seamless steel pipe

Info

Publication number: CN115552049B
Application number: CN202180034296.7A
Authority: CN
Inventors: 藤村和树; 佐佐木俊辅; 柚贺正雄
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2020-06-02
Filing date: 2021-05-11
Publication date: 2023-10-20
Anticipated expiration: 2041-05-11
Also published as: EP4137590A1; EP4137590A4; JPWO2021246118A1; BR112022022689A2; AR122212A1; WO2021246118A1; CN115552049A; JP7004118B1; US20230151469A1; MX2022014264A

Abstract

The present invention provides excellent duplex stainless steel and a duplex stainless steel seamless steel pipe having high strength, high toughness and excellent corrosion resistance. Has the following components in percentage by mass: 0.002-0.03%, si:0.05 to 1.0 percent of Mn:0.10 to 1.5 percent of P:0.040% or less, S:0.0005 to 0.020%, cr:20.0 to 28.0 percent of Ni:4.0 to 10.0 percent of Mo:2.0 to 5.0 percent of Al:0.001 to 0.05 percent and N:0.06 to 0.35%, the balance being Fe and unavoidable impurities, has a structure containing 20 to 70% by volume of an austenite phase and 30 to 80% by volume of a ferrite phase, has a yield strength YS of 448MPa or more, and has a number density of 15 oxide inclusions having an average particle diameter of 1 μm or more per mm ² The proportion of the oxide inclusion containing Al in the oxide inclusion is 50 mass% or less.

Description

Duplex stainless steel and duplex stainless steel seamless steel pipe

Technical Field

The present invention relates to duplex stainless steel and a duplex stainless steel seamless steel pipe which are suitable for use as oil country tubular goods and which have excellent corrosion resistance, high strength and high toughness, and more particularly to duplex stainless steel and a duplex stainless steel seamless steel pipe used for oil country tubular goods.

Background

In recent years, from the viewpoints of an increase in crude oil price and depletion of petroleum resources which can be expected in the near future, development of oil fields, gas fields, and the like, which are deep, which have been conventionally not conceivable, and severe corrosive environments containing hydrogen sulfide and the like, so-called acidic environments, have been actively conducted. Such oil and gas fields are typically extremely deep and the atmosphere is also at a high temperature and contains CO ₂ And Cl ^- And H ₂ Severe corrosive environment of S. Steel pipes for oil wells used in such environments are required to be excellent in strength, toughness, and corrosion resistance (resistance to carbon dioxide corrosion, sulfide stress corrosion cracking, and sulfide stress cracking).

In response to this demand, as the steel used for the steel pipe for oil well, duplex stainless steel is exemplified. The duplex stainless steel is excellent in strength characteristics. On the other hand, in order to apply duplex stainless steel to severe corrosive environments containing a large amount of hydrogen sulfide, carbon dioxide, and chloride ions, such as high-depth oil wells, which have been developed in recent years, it is necessary to improve corrosion resistance.

In this regard, for example, patent document 1 discloses a duplex stainless steel excellent in corrosion resistance, which has a PREW value of 40 or more by controlling the content of Cr, mo, N, W.

Further, patent document 2 discloses a duplex stainless steel excellent in corrosion resistance and hot workability by controlling the content of B, ta or the like in addition to the content of Cr, mo, W, N.

Further, non-patent document 1 experimentally shows that: in stainless steel, mnS of inclusions in steel becomes a starting point of localized corrosion (pitting).

Further, patent document 3 discloses the following duplex stainless steel: in order to reduce sulfide-based inclusions in steel, which adversely affect hot workability and corrosion resistance, a CaO crucible and CaO-CaF are used in a vacuum melting furnace ₂ -Al ₂ O ₃ The S content of the slag is reduced to 3 ppm by weight or less.

Further, patent document 4 discloses a duplex stainless steel in which the total content of Ca and Mg and the S content in oxide inclusions are controlled and the inclusion morphology and density are adjusted as a technique for controlling oxide inclusions that become the starting points of pitting. In addition, patent document 4 discloses the following duplex stainless steel: since Al oxides containing Ca, mg, and S in a certain amount or more become local corrosion starting points even in the case of insoluble Al oxides, the occurrence of local corrosion is suppressed by optimally combining slag basicity at the time of reduction treatment, lethal temperature and time in a ladle, and total processing ratio after casting, thereby controlling the size and number of the above-mentioned inclusions.

Prior art literature

Patent literature

Patent document 1: japanese patent laid-open No. 5-132741

Patent document 2: japanese patent laid-open No. 8-170153

Patent document 3: japanese patent laid-open No. 3-291358

Patent document 4: international publication No. 2005/014872

Non-patent literature

Non-patent document 1: wu Tengquan et al ふぇ (む Vol.17 (2012), no.12,858-863)

Disclosure of Invention

Problems to be solved by the invention

As described above, with recent development of oil fields, gas fields, and the like in a severe corrosive environment, it is desired to maintain high strength, high toughness, and excellent corrosion resistance in oil well steel pipes. Here, excellent corrosion resistance means that in addition to excellent resistance to carbon dioxide corrosion, in particular, CO is contained ₂ And Cl ^- And H ₂ S is excellent in resistance to carbon dioxide corrosion at high temperatures of 200 ℃ under severe corrosive environments, sulfide stress corrosion cracking at low temperatures of 80 ℃ (SCC resistance), and sulfide stress cracking at ordinary temperatures of 25 ℃ (SSC resistance).

However, the steels described in patent documents 1 to 4 have a problem that sulfide stress corrosion cracking resistance at a low temperature of 80 ℃ and sulfide stress cracking resistance at a normal temperature of 25 ℃ are not sufficiently considered.

In view of the above problems, an object of the present invention is to provide duplex stainless steel and a duplex stainless steel seamless steel pipe having high strength, high toughness and excellent corrosion resistance.

Here, the excellent corrosion resistance means a corrosion resistance having both excellent resistance to carbon dioxide corrosion, excellent resistance to sulfide stress corrosion cracking (SCC resistance) and excellent resistance to sulfide stress cracking (SSC resistance) even in a severe corrosion environment as described above. The steel pipe obtained from such duplex stainless steel is suitable for use in severe environments such as oil wells and gas wells of crude oil or natural gas.

In the present invention, "high strength" means that the yield strength YS is 65ksi (448 MPa) or more, preferably 95ksi (655 MPa) or more.

In the present invention, "high toughness" means absorption energy vE of a Charpy impact test at-10℃which is low-temperature toughness _-10 Is 40J or more.

In the present invention, "excellent carbon dioxide corrosion resistance" means that the test piece is held in an autoclave in a test solution: 20% by mass of an aqueous NaCl solution (CO at a liquid temperature of 200 ℃ C., 3.0 MPa) ₂ A gas atmosphere), the immersion period was set to 336 hours, in which case the etching rate was 0.125 mm/year or less and no pitting occurred.

In the present invention, "excellent sulfide stress corrosion cracking resistance (SCC resistance)" means that the test piece is held in an autoclave in a test solution: 10% by mass of aqueous NaCl solution (liquid temperature: CO at 80 ℃ C., 2 MPa) ₂ H of gas, 35kPa ₂ S atmosphere), the soaking period was set to 720 hours, and 100% of the additional yield stress was used as the additional stress, in which case the test piece after the test did not crack and no pitting occurred.

In the present invention, the term "excellent sulfide stress cracking resistance (SSC resistance)" means that the test piece is held in the test cell in a test solution: 20% by mass of an aqueous NaCl solution (liquid temperature: CO at 25 ℃ C., 0.07 MPa) ₂ H of gas, 0.03MPa ₂ S atmosphere), the pH was adjusted to 3.5 by adding acetic acid+na acetate, and the dipping period was set to 720 hours, and 90% of the additional yield stress was used as the additional stress, in which case, the test piece after the test did not crack and pitting did not occur.

Means for solving the problems

The present inventors have conducted intensive studies on the influence of inclusions on sulfide stress corrosion cracking resistance in duplex stainless steel in order to achieve the above object. As a result, the following findings were obtained.

1) Among oxides that become the starting points of pitting, the oxide of MgO host dissolves itself in solution impregnation, and is harmless.

2) For Al ₂ O ₃ Inclusion of main body of Al ₂ O ₃ The reaction proceeds with the cathode and the surrounding base material as the anode, and corrosion occurs around the inclusions.

3) By making Al ₂ O ₃ The number density of the inclusions of the main body is low, and the sulfide stress corrosion cracking resistance is improved. In particular, the number density of oxide inclusions having an average particle diameter of 1 μm or more is 15 pieces/mm ² The oxide inclusions containing Al in the oxide inclusions showed good sulfide stress corrosion cracking resistance when the proportion of the oxide inclusions is 50% or less.

The present invention has been completed based on the above-described findings and further studied. Namely, the gist of the present invention is as follows.

[1] A duplex stainless steel having a composition containing, in mass%, C: 0.002-0.03%, si:0.05 to 1.0 percent of Mn:0.10 to 1.5 percent of P:0.040% or less, S:0.0005 to 0.020%, cr:20.0 to 28.0 percent of Ni:4.0 to 10.0 percent of Mo:2.0 to 5.0 percent of Al:0.001 to 0.05 percent and N: 0.06-0.35%, the balance being Fe and unavoidable impurities,

And has a structure containing 20 to 70% by volume of an austenite phase and 30 to 80% by volume of a ferrite phase,

the yield strength YS is more than 448MPa,

the number density of oxide inclusions having an average particle diameter of 1 μm or more is 15 pieces/mm ² In the following the procedure is described,

the proportion of the oxide inclusion containing Al in the oxide inclusion is 50% or less.

[2] The duplex stainless steel according to the above item [1], wherein the duplex stainless steel further comprises one or more groups selected from the following groups A to E in mass% in addition to the above composition.

Group A: selected from the group consisting of W: less than 1.5%, cu:2.0% or less of one or both;

group B: v: less than 0.20%;

group C: selected from Zr: less than 0.50%, B: less than 0.010%, nb: one or more than two of below 0.50%;

group D: selected from REM: less than 0.005%, ca: less than 0.010%, sn: less than 0.20%, mg: one or more than two of below 0.01%;

group E: selected from Ta: less than 0.10%, co: less than 1.0%, sb:1.0% or less of one or two or more kinds of the above.

[3] The duplex stainless steel according to [1] or [2], wherein the yield strength YS is 655MPa or more.

[4] A duplex stainless steel seamless steel pipe having a composition containing, in mass%, C: 0.002-0.03%, si:0.05 to 1.0 percent of Mn:0.10 to 1.5 percent of P:0.040% or less, S:0.0005 to 0.020%, cr:20.0 to 28.0 percent of Ni:4.0 to 10.0 percent of Mo:2.0 to 5.0 percent of Al:0.001 to 0.05 percent and N: 0.06-0.35%, the balance being Fe and unavoidable impurities,

the yield strength YS is more than 448MPa,

[5] The duplex stainless steel seamless pipe according to the above [4], wherein the composition further comprises one or more groups selected from the following groups A to E in mass%.

group B: v: less than 0.20%;

[6] The duplex stainless steel seamless pipe according to the above [4] or [5], which has a yield strength YS of 655MPa or more.

Effects of the invention

According to the present invention, duplex stainless steel and a duplex stainless steel seamless steel pipe having high strength, high toughness and excellent corrosion resistance can be obtained.

The duplex stainless steel seamless steel pipe manufactured by the present invention is applied to a stainless steel seamless steel pipe for oil well, thereby exhibiting a remarkable industrial effect.

Detailed Description

Hereinafter, embodiments of the present invention will be described. The duplex stainless steel is described below, but the duplex stainless steel seamless pipe may have the same structure as the duplex stainless steel.

The duplex stainless steel of the present invention has a composition containing, in mass%, C: 0.002-0.03%, si:0.05 to 1.0 percent of Mn:0.10 to 1.5 percent of P:0.040% or less, S:0.0005 to 0.020%, cr:20.0 to 28.0 percent of Ni:4.0 to 10.0 percent of Mo:2.0 to 5.0 percent of Al:0.001 to 0.05 percent and N:0.06 to 0.35%, the balance being Fe and unavoidable impurities, comprising an austenite phase in volume ratio:20-70% and ferrite phase: 30 to 80% of the structure, a yield strength YS of 448MPa or more, an average particle diameter of 1 μm or more, and a number density of oxide inclusions of 15 pieces/mm ² The proportion of oxide inclusions containing Al in oxide inclusions having an average particle diameter of 1 μm or more is 50% or less.

Composition of duplex stainless steel

The reasons for limiting the range of the composition of the duplex stainless steel of the present invention will be described below. The% related to the content of the components is "mass%.

C：0.002～0.03％

C is an element having an effect of stabilizing an austenite phase and improving strength and low-temperature toughness. In order to achieve a high strength with a yield strength YS of 65ksi or more (448 MPa or more), the C content is set to 0.002% or more. Preferably, the C content is 0.005% or more. On the other hand, if the C content exceeds 0.03%, precipitation of carbide by heat treatment becomes excessive, and corrosion resistance may be adversely affected. Therefore, the C content is set to 0.03% or less. The C content is preferably 0.02% or less. More preferably, the C content is 0.012% or less.

Si：0.05～1.0％

Si is an element that functions as a deoxidizer, and in order to obtain this effect, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more. On the other hand, if the Si content exceeds 1.0%, precipitation of intermetallic compounds by heat treatment becomes excessive, and corrosion resistance of steel is deteriorated. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.7% or less. More preferably 0.6% or less.

Mn：0.10～1.5％

Mn is an element effective as a deoxidizer as in the case of Si described above, and S inevitably contained in steel is fixed as sulfide to improve hot workability. These effects are obtained when the Mn content is 0.10% or more. Therefore, the Mn content is set to 0.10% or more. The Mn content is preferably 0.15% or more, more preferably 0.20% or more. On the other hand, when the Mn content exceeds 1.5%, not only the hot workability is lowered but also the corrosion resistance is adversely affected. Therefore, the Mn content is set to 1.5% or less. The Mn content is preferably 1.0% or less, more preferably 0.5% or less.

P: less than 0.040 percent

P is an element that reduces the corrosion resistance of duplex stainless steel, and if it exceeds 0.040%, the corrosion resistance is significantly reduced. Therefore, the P content is set to 0.040% or less. The P content is preferably 0.020% or less. However, in order to reduce the P content to less than 0.005%, the depuration treatment takes a long time in the process of smelting molten steel, resulting in an increase in the manufacturing cost of duplex stainless steel. Therefore, the P content is preferably set to 0.005% or more.

S：0.0005～0.020％

S is an element that reduces hot workability in the production process of duplex stainless steel, and if S exceeds 0.020%, it causes trouble in the production of duplex stainless steel. Therefore, S is set to 0.020% or less. The S content is preferably 0.010% or less. More preferably, the S content is 0.005% or less. From the viewpoint of preventing an increase in manufacturing cost, the S content is 0.0005% or more.

Cr：20.0～28.0％

Cr is an effective basic component for maintaining corrosion resistance and improving strength. In order to obtain these effects, the Cr content is set to 20.0% or more. In order to further obtain high strength, the Cr content is preferably 21.0% or more, and more preferably 23.0% or more. On the other hand, when the Cr content exceeds 28.0%, the sigma phase is easily precipitated, and the corrosion resistance and toughness are deteriorated. Therefore, the Cr content is set to 28.0% or less. From the viewpoint of toughness, the Cr content is preferably 27.0% or less.

Ni：4.0～10.0％

Ni is an element that stabilizes an austenite phase and is contained to obtain a two-phase structure. When the Ni content is less than 4.0%, austenite transformation becomes unstable, and the volume fraction of ferrite phase becomes excessively large. Therefore, the Ni content is set to 4.0% or more. The Ni content is preferably 4.5% or more. On the other hand, when the Ni content exceeds 10.0%, the main body becomes an austenite phase, and the volume fraction of the austenite phase becomes excessively large. In addition, ni is an expensive element, and thus, economical efficiency is also impaired. Therefore, the Ni content is set to 10.0% or less. The Ni content is preferably 8.0% or less.

Mo：2.0～5.0％

Mo is an element having an effect of improving corrosion resistance of duplex stainless steel, and in particular, contributes to prevention of corrosion by Cl ^– The induced pitting. When the Mo content is less than 2.0%, the effect is not obtained. Therefore, the Mo content is set to 2.0% or more. The Mo content is preferably 2.5% or more. On the other hand, if the Mo content exceeds 5.0%, the sigma phase precipitates, and the toughness and corrosion resistance are reduced. Therefore, the Mo content is set to 5.0% or less. The Mo content is preferably 4.5% or less.

Al：0.001～0.05％

Al is an element that functions as a deoxidizer in the process of melting molten steel that is a raw material of duplex stainless steel, and when the Al content is less than 0.001%, the effect is not obtained. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.005% or more. On the other hand, when the Al content exceeds 0.05%, alumina inclusions tend to precipitate, and the hot workability in the production process of the duplex stainless steel is lowered, and the toughness is also deteriorated. Therefore, the Al content is set to 0.05% or less. The Al content is preferably 0.04% or less.

N：0.06～0.35％

N is known as an element that improves pitting corrosion resistance and contributes to solid solution strengthening in a normal duplex stainless steel, and is actively added to a content of 0.06% or more. However, in the case of performing the aging heat treatment, N is not said to be an element which forms various nitrides and reduces sulfide stress corrosion cracking resistance and sulfide stress cracking resistance at a low temperature of 80 ℃ or less, but the effect becomes remarkable when it is contained in excess of 0.35%. Therefore, the N content is set to 0.35% or less. The N content is preferably 0.34% or less, more preferably 0.32% or less. In order to obtain the target characteristics of the present invention, the N content is preferably set to 0.07% or more. More preferably, the N content is 0.08% or more.

The balance being Fe and unavoidable impurities. The unavoidable impurities include, for example, O (oxygen), and O is allowable when 0.01% or less.

The above components are basic components. In the present invention, one or two or more groups selected from the following groups a to E may be contained in addition to the above-described basic components, if necessary.

group B: v: less than 0.20%;

Group A

W: less than 1.5% (including 0%)

When W exceeds 1.5% and is contained in a large amount, low-temperature toughness may be lowered. Therefore, when W is contained, the W content is set to 1.5% or less. More preferably, the W content is 1.2% or less. In addition, W is an element that improves sulfide stress corrosion cracking resistance and sulfide stress cracking resistance. In order to obtain such an effect, the W content is preferably 0.02% or more. More preferably, the W content is 0.8% or more.

Cu:2.0% or less (including 0%)

When the Cu content exceeds 2.0%, the low-temperature toughness may be lowered. Therefore, in the case of containing Cu, the Cu content is set to 2.0% or less. More preferably, the Cu content is 1.0% or less. Further, cu precipitates fine epsilon-Cu during aging heat treatment, so that the strength is greatly increased, and the protective coating film is made firm to suppress the invasion of hydrogen into steel, thereby improving sulfide stress cracking resistance and sulfide stress corrosion cracking resistance. In order to obtain these effects, the Cu content is preferably set to 0.1% or more. More preferably, the Cu content is 0.2% or more.

Group B

V: less than 0.20% (including 0%)

When V exceeds 0.20%, the low-temperature toughness may be lowered. In addition, when the content is large, sulfide stress cracking resistance may be reduced. Therefore, when V is contained, the V content is set to 0.20% or less. More preferably, the V content is 0.08% or less. V is an element that increases the strength of steel by precipitation strengthening. In order to obtain such an effect, the V content is preferably 0.02% or more. More preferably, the V content is 0.04% or more.

Group C

Zr: less than 0.50% (including 0%)

Zr, B and Nb are useful as elements contributing to the strength increase, and may be optionally contained. Zr contributes to the above-described strength increase and also contributes to improvement of sulfide stress corrosion cracking resistance. In order to obtain such an effect, the Zr content is preferably set to 0.02% or more. More preferably, the Zr content is 0.05% or more. On the other hand, when Zr is contained in an amount exceeding 0.50%, the low-temperature toughness may be lowered. Therefore, when Zr is contained, the Zr content is set to 0.50% or less. More preferably, the Zr content is 0.30% or less. The Zr content is more preferably 0.20% or less.

B: less than 0.010% (including 0%)

B is useful as an element contributing to the increase in strength and also contributing to the improvement of hot workability. In order to obtain such an effect, the B content is preferably set to 0.0005% or more. More preferably, the B content is 0.0010% or more. On the other hand, if B exceeds 0.010%, the low-temperature toughness and hot workability may be lowered. Therefore, when B is contained, the B content is set to 0.010% or less. More preferably, the B content is 0.0080% or less. The B content is more preferably 0.0030% or less, and still more preferably 0.0025% or less.

Nb: less than 0.50%

Nb contributes to the above-described strength increase and also contributes to improvement in sulfide stress corrosion cracking resistance. In order to obtain such an effect, the Nb content is preferably set to 0.005% or more. More preferably, the Nb content is 0.01% or more. On the other hand, when Nb is contained in an amount exceeding 0.50%, the low-temperature toughness may be lowered. Therefore, when Nb is contained, the Nb content is set to 0.50% or less. More preferably, the Nb content is 0.20% or less.

Group D

REM: less than 0.005% (including 0%)

REM is useful as an element contributing to improvement of sulfide stress corrosion cracking resistance, and may be contained as needed. In order to ensure such effects, it is preferable to contain 0.001% or more of REM. More preferably, the REM content is 0.0015% or more. On the other hand, even if REM is contained in an amount exceeding 0.005%, the effect is saturated, and an effect corresponding to the content cannot be expected, and sometimes it is economically disadvantageous. Therefore, when the REM content is contained, the REM content is set to 0.005% or less. More preferably, the REM content is 0.004% or less.

In the present invention, REM refers to lanthanoids of scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium (Lu) having an atomic number of 71. The REM concentration in the present invention means the total content of one or more elements selected from the above REM.

Ca: less than 0.010% (including 0%)

Ca is useful as an element contributing to improvement of sulfide stress corrosion cracking resistance, and may be contained as needed. In order to ensure such effects, ca is preferably contained in an amount of 0.001% or more. More preferably, the Ca content is 0.0015% or more. On the other hand, even if the content exceeds 0.010%, the effect is saturated, and the effect corresponding to the content cannot be expected, and sometimes it is economically disadvantageous. Therefore, when the content is contained, the Ca content is set to 0.010% or less. More preferably, the Ca content is 0.0080% or less. The Ca content is more preferably 0.005% or less, and still more preferably 0.004% or less.

Sn: less than 0.20% (including 0%)

Sn is useful as an element contributing to improvement of sulfide stress corrosion cracking resistance, and may be contained as needed. In order to ensure such effects, it is preferable to contain 0.05% or more of Sn. More preferably, the Sn content is 0.09% or more. On the other hand, even if Sn is contained in an amount exceeding 0.20%, the effect is saturated, and an effect corresponding to the content cannot be expected, and sometimes it is economically disadvantageous. Therefore, in the case of the content, the Sn content is set to 0.20% or less. More preferably, the Sn content is 0.15% or less.

Mg: less than 0.01% (including 0%)

Mg is useful as an element contributing to improvement of sulfide stress corrosion cracking resistance, and may be contained as needed. Even if Mg is contained in an amount exceeding 0.01%, the effect becomes saturated, and an effect corresponding to the content cannot be expected, which sometimes becomes economically disadvantageous. Therefore, in the case of the inclusion, the Mg content is set to 0.01% or less. More preferably, the Mg content is 0.008% or less. Further preferably, the Mg content is 0.005% or less. In order to ensure the above effect, mg is preferably contained in an amount of 0.0002% or more. More preferably, the Mg content is 0.0005% or more.

Group E

Ta: less than 0.10% (including 0%)

Ta is useful as an element contributing to improvements in carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be contained as needed. Even if Ta is contained in an amount exceeding 0.10%, the effect is saturated, and an effect corresponding to the content may not be expected. Therefore, in the case of the content, the Ta content is set to 0.10% or less. More preferably, the Ta content is 0.05% or less. In order to ensure the above effect, it is preferable to contain 0.01% or more of Ta. More preferably, the Ta content is 0.02% or more.

Co: less than 1.0% (including 0%)

Co is useful as an element contributing to improvements in carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be contained as needed. Even if Co is contained in an amount exceeding 1.0%, the effect is saturated, and an effect corresponding to the content may not be expected. Therefore, when the Co content is contained, the Co content is set to 1.0% or less. More preferably, the Co content is 0.5% or less. The Co content is more preferably 0.1% or less. In order to ensure the above-described effect, it is preferable to contain 0.01% or more of Co. More preferably, the Co content is 0.02% or more.

Sb: less than 1.0% (including 0%)

Sb is useful as an element contributing to improvement of carbon dioxide corrosion resistance, sulfide stress cracking resistance and sulfide stress corrosion cracking resistance, and may be contained as needed. Even if Sb is contained in an amount exceeding 1.0%, the effect is saturated, and an effect corresponding to the content may not be expected. Therefore, when the content is contained, the Sb content is set to 1.0% or less. More preferably, the Sb content is 0.5% or less. Further preferably, the Sb content is 0.1% or less. In order to ensure the above effect, sb is preferably contained in an amount of 0.01% or more. More preferably, the Sb content is 0.02% or more.

Double-phase stainless steel structure

Contains austenite phase in volume percent: 20-70% and ferrite phase: 30-80% of tissue

The duplex stainless steel of the present invention has a structure containing at least an austenite phase and a ferrite phase, and may have a structure composed of the austenite phase and the ferrite phase. The volume percent (%) of the austenite phase is 20 to 70%. The volume ratio (%) of the ferrite phase is 30 to 80%. If the austenite phase is less than 20%, the low-temperature toughness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance may be poor. In addition, when the austenite phase exceeds 70%, the strength may be poor. In addition, when the ferrite phase exceeds 80%, the low-temperature toughness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance may be poor. In addition, when the ferrite phase is less than 30%, the strength may be poor.

The volume fraction of each phase can be controlled by adjusting the steel composition and the solution treatment temperature. Specifically, the more the austenite phase forming element (C, mn, ni, N, cu, co) or the lower the solution treatment temperature, the more the volume fraction of the austenite phase increases. The more the ferrite phase forming element (Si, cr, mo, W) or the higher the solution treatment temperature, the more the volume fraction of the ferrite phase increases.

As a method for measuring the volume fractions of the respective phases, first, a test piece for tissue observation was cut out with the cross section of the duplex stainless steel as an observation surface (in the case where the duplex stainless steel is a seamless steel pipe, the cross section in the pipe axis direction is an observation surface). Then, the volume ratios of the ferrite phase and the austenite phase were obtained by observing the observation surface with a Scanning Electron Microscope (SEM). Specifically, the above-mentioned test piece for tissue observation was corroded with a Villella's solution (a reagent obtained by mixing picric acid, hydrochloric acid and ethanol in a ratio of 2g, 10ml and 100ml, respectively), and the tissue was photographed by a scanning electron microscope (1000-fold). From the obtained tissue photograph, an average value of area ratios of ferrite phase and austenite phase was calculated using an image analysis device, and the average value was defined as each volume ratio (vol%). In the captured image, a phase which is white by binarization and is less likely to be corroded is set as a ferrite phase, and a phase which is black by binarization and is likely to be corroded is set as an austenite phase.

Oxide-based inclusions

Number density of oxide inclusions having an average particle diameter of 1 μm or more: 15 pieces/mm ² The following are the following

In the present invention, the number density of oxide inclusions having an average particle diameter of 1 μm or more is set to 15 inclusions/mm ² The following is given. Oxide inclusions having an average particle diameter of 1 μm or more are dissolved in a corrosive environment and tend to become points of initiation of pitting. On the other hand, even if the oxide inclusions having an average particle diameter of less than 1 μm are dissolved, the gaps formed between the oxide inclusions and the base material are small, and the oxide inclusions do not develop into pitting.

The number density of oxide inclusions having an average particle diameter of 1 μm or more exceeds 15 inclusions/mm ² In this case, at least one of good pitting resistance, SSC resistance and SCC resistance cannot be obtained. Accordingly, in the present invention, the number density of oxide inclusions having an average particle diameter of 1 μm or more is set to 15 pieces/mm ² The following is given. Preferably 13 pieces/mm ² Hereinafter, more preferably 10 pieces/mm ² The following is given.

The content of oxide inclusions containing Al is 50% or less

In the present invention, the proportion of the oxide inclusions containing Al in the oxide inclusions having an average particle diameter of 1 μm or more is set to 50% or less.

When the proportion of the oxide inclusion containing Al exceeds 50%, at least one of good pitting corrosion resistance, SSC resistance, and SCC resistance cannot be obtained. Therefore, the proportion of oxide inclusions containing Al in oxide inclusions having an average particle diameter of 1 μm or more is set to 50% or less. Preferably 48% or less, and more preferably 45% or less.

As a method for measuring the number density of oxide inclusions, first, a test piece similar to the above-described test piece for tissue observation was mirror polished, and SEM observation of 5 fields of view was performed at a magnification of 50 times. Then, the vicinity of the center portion of the oxide inclusion was subjected to composition analysis by EDX (energy dispersive X-ray analysis). As an element at the time of analysis, the mass ratio of Al, ca, mg, S, mn was measured. As measurement conditions, it is preferable to perform electron beam irradiation at a sufficiently large acceleration voltage (for example, 15 kV) in order to reduce variation in analysis values. When the irradiation current is too large, the SEM image resolution is lowered, and when it is too small, the amount of X-ray generated necessary for measurement is not obtained, so that it is preferable to perform the measurement with an appropriate irradiation current amount.

The average particle diameter of the oxide-based inclusions is obtained by measuring the long and short diameters of the inclusions and averaging them (the sum of the long and short diameters divided by 2).

Further, the oxide inclusion containing 20 mass% or more of Al was determined as "oxide inclusion containing Al", and the number density of oxide inclusions having an average particle diameter of 1 μm or more and the proportion of oxide inclusions containing Al in the oxide inclusions were derived.

The number density of oxide inclusions having an average grain size of 1 μm or more in the duplex stainless steel of the present invention can be controlled by the vacuum stirring time after the Al is added in the deoxidizing step of steel making.

Method for manufacturing duplex stainless steel

Hereinafter, a method for producing the duplex stainless steel of the present invention will be described. The duplex stainless steel of the present invention can be applied not only to a seamless steel pipe but also to a stainless steel plate, a UOE steel pipe using the same, an ERW steel pipe, a spiral steel pipe, a forge welded pipe, and the like.

In the present invention, a steel material such as a billet having the above-described composition is used as a starting material (hereinafter, may be referred to as a steel pipe material). In the present invention, the method for producing the starting material is not particularly limited, and a generally known production method can be applied.

As a method for producing a steel pipe material having the above-described composition, for example, a steel pipe material is produced by melting molten steel having the above-described composition by a usual melting method such as a converter, and by a usual known method such as a continuous casting method or an ingot-cogging rolling method.

In the deoxidizing step of the steelmaking step, in order to reduce the amount of Al inclusions in the steel, a method of vacuum stirring after adding Al to separate Al inclusions in a floating manner is used.

In order to sufficiently float and separate the inclusions, it is preferable to set the temperature of the molten steel to 1550 ℃ or higher and perform vacuum stirring. As a method for increasing the temperature of molten steel, there may be mentioned: in the deoxidation step or the decarburization step before the deoxidation step, oxygen blowing is performed. Al is produced by reacting oxygen in molten steel having increased in oxygen blowing with Al charged in the deoxidizing step ₂ O ₃ 。

In order to achieve a desired number density of oxide inclusions and a desired proportion of oxide inclusions containing Al, it is necessary to sufficiently float the inclusions, and therefore, the vacuum stirring time is set to 15 minutes or more, calculated from the operation later in oxygen blowing and Al input.

The vacuum stirring time is preferably set to 60 minutes or less in order to prevent the decrease in the temperature of the molten steel.

Next, these steel pipe materials are heated, and a seamless steel pipe having the above-described composition of a desired size is produced by a hot working such as an extrusion pipe making method, a mannesmann pipe making method, or the like, which is a generally known pipe making method.

Solution heat treatment

Then, the steel pipe after the pipe formation is subjected to solution heat treatment. Specifically, the steel pipe is heated to a heating temperature of 1000 ℃ or higher and then cooled to a temperature of 300 ℃ or lower at an average cooling rate of air cooling or higher, more specifically at an average cooling rate of 1 ℃/s or higher. Thus, intermetallic compounds, carbides, nitrides, sulfides and the like precipitated during the tube making or during cooling after the tube making can be solid-dissolved to produce a seamless steel tube having a desired amount of an austenite phase and ferrite phase structure.

When the heating temperature of the solution heat treatment is lower than 1000 ℃, the desired high toughness cannot be ensured. The heating temperature of the solution heat treatment is preferably 1020 ℃ or higher. In addition, from the viewpoint of preventing the volume fraction of ferrite phase from becoming excessive, the heating temperature of the solution heat treatment is set to 1200 ℃ or lower. The heating temperature of the solution heat treatment is preferably 1150 ℃ or lower. More preferably, the solution heat treatment has a heating temperature of 1130 ℃ or lower. In the present invention, from the viewpoint of making the temperature in the material uniform, the holding time at the heating temperature of the solution heat treatment is preferably 5 minutes or longer. The holding time at the heating temperature of the solution heat treatment is preferably 210 minutes or less.

When the average cooling rate of the solution heat treatment is less than 1 ℃/s, intermetallic compounds such as sigma phase and χ phase are precipitated during cooling, and low-temperature toughness and corrosion resistance are remarkably reduced. The upper limit of the average cooling rate is not particularly limited. The cooling rate of the cooling in the solution heat treatment is preferably 2 ℃/s or more.

Cold working

In order to increase the yield strength of the material, strain may be introduced by cold drawing, cold pilger rolling, or cold-shrink radial rolling by oblique rolling of the opposing rolls, and the material may be strengthened. Preferably, the reduction rolling is performed. As the tilt rolling mill used in the reducing rolling, a two-roll type tilt rolling mill having barrel rolls and a three-roll type tilt rolling mill can be used. The reducing rolling can be performed by adjusting the inclination angle, the intersection angle and the roll gap. The temperature at the time of processing may be high in order to reduce deformation resistance. Specifically, the working strengthening temperature is preferably in the range of 25 to 600 ℃ and in the temperature range of 460 to 490 ℃ avoiding embrittlement of the stainless steel.

Further, after cold working, an aging heat treatment for improving the yield strength by age hardening may be performed. When the aging heat treatment temperature exceeds 700 ℃, intermetallic compounds such as sigma phase and χ are separated out, and the low-temperature toughness and corrosion resistance are obviously reduced. Therefore, the aging heat treatment temperature is preferably set to 700 ℃.

The duplex stainless steel of the present invention has been described above, but the duplex stainless steel seamless steel pipe of the present invention may have the same constitution as that of the duplex stainless steel.

According to the present invention, a duplex stainless steel and a duplex stainless steel seamless steel pipe having a high strength with a yield strength YS of 65ksi (448 MPa) or more, preferably 95ksi (655 MPa) or more, and high toughness and excellent corrosion resistance can be obtained.

Examples

The following describes embodiments of the present invention. The present invention is not limited to the following examples.

Molten steel having the composition shown in table 1 was melted in a converter. The number and composition of oxide inclusions were adjusted by changing the vacuum stirring time in the Al killed steel shown in table 2.

Then, a billet (steel pipe raw material) was cast by a continuous casting method, and after the steel pipe raw material was heated at 1150 to 1250 ℃, a pipe was manufactured by hot working using a heated die seamless rolling mill, whereby a seamless steel pipe having an outer diameter of 62mm and a wall thickness of 7mm, or a seamless steel pipe having an outer diameter of 131mm and a wall thickness of 25mm was manufactured.

The obtained seamless steel pipe was air-cooled after pipe production. After air cooling, solution heat treatment was performed at the temperature shown in table 2 for 30 minutes.

The average cooling rate in the solution heat treatment was set to 2 ℃/s.

The steel pipe partially solution heat treated is subjected to reduction rolling or cold pilger rolling as work strengthening. Reducing rolling is carried out on the steel pipe with the outer diameter of 62mm and the wall thickness of 7mm, and cold pilger rolling is carried out on the seamless steel pipe with the outer diameter of 131mm and the wall thickness of 25 mm.

The tilt rolling mill used in the reducing rolling was a two-roll type tilt rolling mill (see two rolls in table 2) or a three-roll type tilt rolling mill (see three rolls in table 2) having barrel rolls with an entrance side angle of 2.5 ° and an exit side angle of 3.0 ° when the tilt angle and the intersection angle were both 0 °, and the reducing rolling was performed by adjusting the tilt angle to 6 ° and the intersection angle to 0 ° at the time of rolling, and setting the roll gap to 56 mm. In cold working, rolling was performed by cold pilger method at a reduction rate of 70% (see cold pilger in table 2). As for the temperature at the time of processing (refer to the processing strengthening temperature in table 2), a part is performed at a high temperature in order to reduce deformation resistance. Specifically, the working strengthening temperature is set to 25 ℃ or 500 ℃ in order to conduct working in a temperature range of 25 to 600 ℃ and avoiding the embrittlement of stainless steel in a temperature range of 460 to 490 ℃.

Further, a part of the steel pipe (steel pipe with a heating temperature of the aging heat treatment is described in table 2) was subjected to the aging heat treatment.

From the finally obtained seamless steel pipe, a test piece for observing a structure was cut out, and quantitative evaluation of the structure, a tensile test, a Charpy impact test, a corrosion test, a sulfide stress cracking test (SSC test) and a sulfide stress corrosion cracking test (SCC test) were performed. The test method is as follows. The results obtained by these experiments are shown in table 2.

(1) Determination of the volume fraction (vol%) of each phase in the entire structure of the steel pipe

From the seamless steel pipe subjected to the heat treatment, a test piece for tissue observation was cut so that the cross section in the pipe axis direction was an observation surface. The volume fractions of the ferrite phase and the austenite phase were obtained by observing the observation surface with a scanning electron microscope. Specifically, the test piece for tissue observation was corroded with a vella reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol at a ratio of 2g, 10ml, and 100ml, respectively), and the tissue was photographed by a Scanning Electron Microscope (SEM) (1000 times). From the obtained tissue photograph, an average value of area ratios of ferrite phase and austenite phase was calculated using an image analysis device, and the average value was defined as each volume ratio (vol%).

In the captured image, a phase which is white by binarization and is less likely to be corroded is set as a ferrite phase, and a phase which is black by binarization and is likely to be corroded is set as an austenite phase. In the binarization, a 256-level gray-scale image is formed on the captured image, and then the image is performed over a measurement region (600 μm×800 μm (1920 pixels×2560 pixels)). Regarding the setting of binarization, the minimum luminance between two peaks seen in a histogram in which the horizontal axis is set as luminance (256 levels) is set as a threshold value.

(2) Number density measurement of oxide inclusions

The same test piece as the above-described test piece for tissue observation was mirror polished, and SEM observation was performed for each 5 fields at 50 times magnification. Then, the vicinity of the center portion of the oxide inclusion was subjected to composition analysis by EDX (energy dispersive X-ray analysis). As an element at the time of analysis, the mass ratio of Al, ca, mg, S, mn was measured. As measurement conditions, it is preferable to perform electron beam irradiation at a sufficiently large acceleration voltage (15 kV) in order to reduce variation in analysis values.

The oxide containing 20 mass% or more of Al was determined as "oxide containing Al", and the number density (density of inclusions (units/mm in Table 2) was derived with an average particle diameter of 1 μm or more ² ) And the proportion of oxide inclusions containing Al therein (in Table 2, the proportion (%) of oxide containing Al). The average particle diameter is obtained by measuring the major axis and the minor axis of the inclusion and averaging them.

(3) Tensile test

From the seamless steel pipe subjected to the heat treatment, an API arc tensile test piece was cut so that the tensile direction was the tube axis direction according to the API-5CT standard. For the test pieces thus cut, tensile tests were conducted according to the API standard, and the yield strength YS (MPa) and the tensile strength TS (MPa) were measured as tensile characteristics.

(4) Charpy impact test

From the center of the wall thickness of the seamless steel pipe subjected to the heat treatment, a V-shaped notch was cut out according to ISO-11960 so that the length direction of the pipe would be the length of the test pieceOral test piece (thickness 5 mm). For the test piece thus cut, the Charpy impact test was performed at a test temperature of-10℃to measure the absorption energy vE _-10 (J) A. The invention relates to a method for producing a fibre-reinforced plastic composite 3 test pieces were cut from each steel pipe, and these test pieces were subjected to the Charpy impact test, and the arithmetic average of the obtained values is shown in Table 2.

(5) Corrosion test (carbon dioxide Corrosion resistance test)

From the seamless steel pipe subjected to the heat treatment, corrosion test pieces having a thickness of 3mm×a width of 30mm×a length of 40mm were produced by machining, and the corrosion test was performed on these test pieces to evaluate the carbon dioxide corrosion resistance.

In the corrosion test, the test piece was held in an autoclave with a test liquid: 20 mass% NaCl aqueous solution (liquid temperature: 200 ℃ C., CO) ₂ :3.0 MPa) and the immersion period was set to 14 days (336 hours), the mass of the test piece after the test was measured, and the corrosion rate was calculated from the mass decrease before and after the corrosion test. In addition, for the test piece after the corrosion test, the rate was used: and a magnifying glass of 10 times, and observing whether pitting corrosion occurs on the surface of the test piece. Pitting refers to the presence of a diameter when the pitting is assumed to be circular: and 0.2mm or more. In the present invention, the case where the corrosion rate was 0.125 mm/year or less and no pitting occurred was evaluated as acceptable. In table 2, the case where no pitting occurred is indicated by a symbol o, and the case where pitting occurred is indicated by a symbol x.

(6) Sulfide stress cracking test (SSC test)

From the seamless steel pipe subjected to the heat treatment, round bar-shaped test pieces (diameter: 6.4 mm. Phi.) were produced by machining according to NACE TM0177 method A, and SSC resistance test was performed on these test pieces.

In the SSC resistance test, a test piece was subjected to a test solution: 20% by mass of aqueous NaCl solution (liquid temperature: 25 ℃ C., H) ₂ S：0.03MPa、CO ₂ : an atmosphere of 0.07 MPa) was immersed in an aqueous solution having a pH of 3.5 adjusted by adding acetic acid+Na acetate, and the immersion period was set to 720 hours90% of the additional yield stress is implemented as additional stress. The test piece after the test was visually inspected for the presence or absence of cracking. In addition, for the test piece after the test, the multiplying power was used: the presence or absence of pitting on the test piece surface was observed with a magnifying glass at a magnification of 10 times. In the present invention, the test piece after the test was evaluated as being qualified in the case where no crack occurred and no pitting occurred. In table 2, the case where no cracking occurred and no pitting occurred is indicated by a symbol o, and the case where cracking occurred and/or the case where pitting occurred is indicated by a symbol x.

(7) Sulfide stress corrosion cracking test (SCC resistance test)

From the seamless steel pipe subjected to the heat treatment described above, a 4-point bending test piece having a thickness of 3mm×a width of 15mm×a length of 115mm was cut by machining, and the SCC resistance test was performed on the test piece.

In the SCC resistance test, a test piece was placed in a test solution held in an autoclave: 10% by mass aqueous NaCl solution (liquid temperature: 80 ℃ C., H) ₂ S：35kPa、CO ₂ : an atmosphere of 2 MPa), and the soaking period was set to 720 hours, 100% of the additional yield stress was applied as the additional stress. The test piece after the test was visually inspected for the presence or absence of cracking on the surface of the test piece. In addition, for the test piece after the test, the multiplying power was used: the presence or absence of pitting on the test piece surface was observed with a magnifying glass at a magnification of 10 times. In the present invention, the test piece after the test was evaluated as being qualified in the case where no crack occurred and no pitting occurred. In table 2, the case where no cracking occurred and no pitting occurred is indicated by a symbol o, and the case where cracking occurred and/or the case where pitting occurred is indicated by a symbol x.

TABLE 1

^* The balance other than the above components is Fe and unavoidable impurities.

^* Underline is hairOutside the clear range.

TABLE 2

^* Underlined is outside the scope of the invention.

The inventive examples all formed a steel sheet with yield strength: absorption energy vE of high strength of 448MPa or more and Charpy impact test _-10 High toughness of not less than 40J and CO content ₂ And Cl ^– Has excellent corrosion resistance (resistance to carbon dioxide corrosion) in a high-temperature corrosive environment of 200 ℃ or higher, and contains H ₂ S does not crack (SSC and SCC) even in the environment of S, and has excellent sulfide stress cracking resistance and sulfide stress corrosion cracking resistance. On the other hand, in the comparative examples which deviate from the scope of the present invention, the high strength as the object of the present invention was not achieved, or the high toughness was not achieved, or H was contained ₂ Cracking (SSC and/or SCC) occurs in the environment of S.

Claims

1. A duplex stainless steel having a composition containing, in mass%, C: 0.002-0.03%, si:0.05 to 1.0 percent of Mn:0.10 to 1.5 percent of P:0.040% or less, S:0.0005 to 0.020%, cr:20.0 to 28.0 percent of Ni:4.0 to 10.0 percent of Mo:2.0 to 5.0 percent of Al:0.001% or more and less than 0.05% and N: 0.06-0.35%, the balance being Fe and unavoidable impurities,

the yield strength YS is more than 448MPa,

2. The duplex stainless steel according to claim 1, further comprising one or more of the following groups a to E in mass% based on the composition of the components:

group B: v: less than 0.20%;

3. The duplex stainless steel according to claim 1 or 2, having a yield strength YS of 655MPa or more.

4. A duplex stainless steel seamless steel pipe having a composition containing, in mass%, C: 0.002-0.03%, si:0.05 to 1.0 percent of Mn:0.10 to 1.5 percent of P:0.040% or less, S:0.0005 to 0.020%, cr:20.0 to 28.0 percent of Ni:4.0 to 10.0 percent of Mo:2.0 to 5.0 percent of Al:0.001% or more and less than 0.05% and N: 0.06-0.35%, the balance being Fe and unavoidable impurities,

the yield strength YS is more than 448MPa,

5. The duplex stainless steel seamless steel pipe according to claim 4, further comprising one or more of the following groups a to E in mass% based on the composition of the components:

group B: v: less than 0.20%;

6. The duplex stainless steel seamless steel pipe according to claim 4 or 5, having a yield strength YS of 655MPa or more.